U.S. patent application number 14/233347 was filed with the patent office on 2014-05-15 for wide-angle lens and imaging device.
The applicant listed for this patent is Daisuke Bessho, Toru Harada, Nozomi Imae, Yoshiaki Irino, Kensuke Masuda, Hiroyuki Satoh, Satoshi Sawaguchi, Hirokazu Takenaka, Tomonori Tanaka, Noriyuki Terao, Hideaki Yamamoto. Invention is credited to Daisuke Bessho, Toru Harada, Nozomi Imae, Yoshiaki Irino, Kensuke Masuda, Hiroyuki Satoh, Satoshi Sawaguchi, Hirokazu Takenaka, Tomonori Tanaka, Noriyuki Terao, Hideaki Yamamoto.
Application Number | 20140132709 14/233347 |
Document ID | / |
Family ID | 47601260 |
Filed Date | 2014-05-15 |
United States Patent
Application |
20140132709 |
Kind Code |
A1 |
Satoh; Hiroyuki ; et
al. |
May 15, 2014 |
WIDE-ANGLE LENS AND IMAGING DEVICE
Abstract
A wide-angle lens having a field angle larger than 180 degrees
includes, in order from an object side to an image side, a front
group, a reflection surface, and a back group, wherein the front
group includes three lenses having a negative refractive power, the
reflection surface is configured to curve an optical axis of the
front group at 90 degrees toward the back group, the back group
includes four lenses having a positive refractive power, a front
principle point is set between a second lens and a third lens from
the object side in the front group, and a focal length of an entire
system f and a distance between an intersection of the reflection
surface and the optical axis of the front group and the front
principle point d satisfy the following condition (1)
7.0<d/f<9.0.
Inventors: |
Satoh; Hiroyuki;
(Kawasaki-shi, JP) ; Terao; Noriyuki; (Sendai-shi,
JP) ; Irino; Yoshiaki; (Kawasaki-shi, JP) ;
Tanaka; Tomonori; (Yokohama-shi, JP) ; Imae;
Nozomi; (Yokohama-shi, JP) ; Harada; Toru;
(Yokohama-shi, JP) ; Takenaka; Hirokazu;
(Kawasaki-shi, JP) ; Yamamoto; Hideaki;
(Yokohama-shi, JP) ; Masuda; Kensuke;
(Kawasaki-shi, JP) ; Sawaguchi; Satoshi;
(Yokohama-shi, JP) ; Bessho; Daisuke;
(Kawasaki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Satoh; Hiroyuki
Terao; Noriyuki
Irino; Yoshiaki
Tanaka; Tomonori
Imae; Nozomi
Harada; Toru
Takenaka; Hirokazu
Yamamoto; Hideaki
Masuda; Kensuke
Sawaguchi; Satoshi
Bessho; Daisuke |
Kawasaki-shi
Sendai-shi
Kawasaki-shi
Yokohama-shi
Yokohama-shi
Yokohama-shi
Kawasaki-shi
Yokohama-shi
Kawasaki-shi
Yokohama-shi
Kawasaki-shi |
|
JP
JP
JP
JP
JP
JP
JP
JP
JP
JP
JP |
|
|
Family ID: |
47601260 |
Appl. No.: |
14/233347 |
Filed: |
July 23, 2012 |
PCT Filed: |
July 23, 2012 |
PCT NO: |
PCT/JP2012/069267 |
371 Date: |
January 16, 2014 |
Current U.S.
Class: |
348/36 ;
359/733 |
Current CPC
Class: |
G02B 19/008 20130101;
G02B 13/06 20130101; G02B 27/1066 20130101; G02B 13/0065 20130101;
H04N 5/23238 20130101 |
Class at
Publication: |
348/36 ;
359/733 |
International
Class: |
G02B 13/00 20060101
G02B013/00; H04N 5/232 20060101 H04N005/232; G02B 13/06 20060101
G02B013/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 25, 2011 |
JP |
2011-162213 |
Claims
1. A wide-angle lens having a field angle larger than 180 degrees,
comprising: in order from an object side to an image side, a front
group; a reflection surface; and a back group, wherein the front
group includes three lenses having a negative refractive power, the
reflection surface is configured to curve an optical axis of the
front group at 90 degrees toward the back group, the back group
includes four lenses having a positive refractive power, a front
principle point is set between a second lens and a third lens from
the object side in the front group, and a focal length of an entire
system f and a distance between an intersection of the reflection
surface and the optical axis of the front group and the front
principle point d satisfy the following condition (1).
7.0<d/f<9.0 (1)
2. The wide-angle lens according to claim 1, wherein a distance
from a most object side surface of the front group to the
reflection surface DA and a distance from the reflection surface to
a most image side surface of the back group DB satisfy the
following condition (2). DA<DB (2)
3. The wide-angle lens according to claim 1, wherein the reflection
surface arranged between the front group and the back group is an
inclined surface of a right angle prism, and is configured to
internal-reflect a light beam from the front group toward the back
group, and a reflective index relative to d-line of a material of
the right angle prism nd satisfies the following condition (3).
nd.gtoreq.1.8 (3)
4. The wide-angle lens according to claim 3, wherein the front
group includes, in order from the object side, a negative meniscus
lens, a negative lens made of a plastic material and a negative
meniscus lens, the back group includes, in order from the object
side, a biconvex lens, a cemented lens of a biconvex lens and a
biconcave lens and a biconvex lens made of a plastic material, an
aperture stop is arranged between the right angle prism and the
back group, the negative lens made of the plastic material in the
front group and the biconvex lens made of the plastic material in
the back group have aspheric surfaces on both surfaces, and the
other lenses except the lenses made of the plastic material are
spherical lenses, respectively.
5. An imaging device comprising two imaging optical systems
including a wide-angle lens having a field angle wider than 180
degrees and an imaging sensor which images an image by the
wide-angle lens, the two imaging optical systems being combined
such that object side lenses are opposed to each other, and the
images by the respective imaging optical systems being synthesized
to obtain an image in a solid angle of 4.pi. radian, wherein the
wide-angle lens for use in each of the two imaging optical systems
is the wide-angle lens according to claim 1, and a distance between
an intersection of the reflection surface and the optical axis of
the front group and a front principle point in the wide-angle lens
of one imaging optical system d1, a distance between an
intersection of the reflection surface and the optical axis of the
front group and the front principle point in the wide-angle lens of
the other imaging optical system d2 and a focal length of the
wide-angle lens of each imaging optical system f satisfy the
following condition (4). 16.ltoreq.(d1+d2)/f<21 (4)
6. The imaging device according to claim 5, wherein the wide-angle
lenses for use in the two imaging optical systems are d1=d2.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application is based on and claims priority from
Japanese Patent Application No. 2011-162213, filed on, Jul. 25,
2011, the disclosure of which is hereby incorporated by reference
in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to a wide-angle lens for use
in an imaging device and an imaging device.
BACKGROUND ART
[0003] An imaging device in which two imaging optical systems each
including a wide-angle lens having a field angle wider than 180
degrees and an imaging sensor for imaging an image by the
wide-angle lens are combined such that the respective object side
lenses are opposed to each other, and the images by the respective
imaging optical systems are synthesized to obtain an image in a
solid angle of 4.pi. radian is known (refer to Japanese Patent
Publication No. 3290993).
[0004] Such an imaging device can simultaneously obtain image
information in all directions; thus, it can be effectively used for
a security monitoring camera or a car-mounted camera, for example.
In recent years, it is required to downsize such an imaging device
to be used as a portable imaging device.
[0005] Extremely accurate and fair image information can be
obtained by using a small imaging device in a hand-held condition
in report of news, for example.
[0006] It is preferable to deflect the light of the maximum image
height away from the optical axis in the wide-angle lens having a
field angle of 180 degrees or more for use in such an imaging
device without using a sharp angle to be imaged on an imaging
surface.
[0007] However, if the entire length of the wide-angle lens having
a field angle of 180 degrees or more is reduced, it becomes
necessary to drastically deflect the light beam in the periphery
away from the optical axis. For this reason, a resolution in the
peripheral portion of the imaging surface is reduced due to various
aberrations.
[0008] It is necessary to gently deflect the light beam in the
periphery for maintaining a high resolution in the periphery of the
imaging surface. For this reason, the entire length of the lens is
increased, and such an imaging device is not suitable for use in a
hand-held condition.
[0009] Japanese Patent Publication No. 3290993 does not
specifically describe a wide-angle lens.
[0010] Various wide-angle lenses having a wide field angle and a
good performance have been conventionally proposed. Among them, the
wide-angle lenses described in Japanese Patent Application
Publication Nos. 2007-155977 and 2010-256627 specially have a good
performance.
[0011] However, it is difficult to reduce the entire length of the
wide-angle lens described in Japanese Patent Application
Publication Nos. 2007-155977 and 2010-256627. If such wide-angle
lenses are used as two wide-angle lenses for use in an imaging
device, the size of the device is increased.
[0012] When using two wide-angle lenses as described in Japanese
Patent Application Publication Nos. 2007-155977 and 2010-256627, it
is difficult to reduce the distance between the optical axes of the
two wide-angle lenses, and the overlapped portions of the images in
the peripheral portions of the respective wide-angle lenses are
misaligned to each other due to disparity. Thus, deterioration in
an image is likely to remarkably develop in the jointed portion of
the synthetic image.
SUMMARY OF THE INVENTION
[0013] The present invention has been made in view of the above
circumferences. An object of the present invention is to provide a
wide-angle lens for use in an imaging device, which can obtain a
synthetic image having less disparity and can ensure an optical
performance.
[0014] An object of the present invention is also to provide a
small imaging device using the above two wide-angle lenses.
[0015] In order to achieve the above object, one embodiment of the
present invention provides a wide-angle lens having a field angle
larger than 180 degrees, including: in order from an object side to
an image side, a front group; a reflection surface; and a back
group, wherein the front group includes three lenses having a
negative refractive power, the reflection surface is configured to
curve an optical axis of the front group at 90 degrees toward the
back group, the back group includes four lenses having a positive
refractive power, a front principle point is set between a second
lens and a third lens from the object side in the front group, and
a focal length of an entire system f and a distance between an
intersection of the reflection surface and the optical axis of the
front group and the front principle point d satisfy the following
condition (1).
7.0<d/f<9.0 (1)
[0016] Preferably, a distance from a most object side surface of
the front group to the reflection surface DA and a distance from
the reflection surface to a most image side surface of the back
group DB satisfy the following condition (2).
DA<DB (2)
[0017] Preferably, the reflection surface arranged between the
front group and the back group is an inclined surface of a right
angle prism, and is configured to internal-reflect a light beam
from the front group toward the back group, and a reflective index
relative to d-line of a material of the right angle prism nd
satisfies the following condition (3).
nd.gtoreq.1.8 (3)
[0018] Preferably, the front group includes, in order from the
object side, a negative meniscus lens, a negative lens made of a
plastic material and a negative meniscus lens, the back group
includes, in order from the object side, a biconvex lens, a
cemented lens of a biconvex lens and a biconcave lens and a
biconvex lens made of a plastic material, an aperture stop is
arranged between the right angle prism and the back group, the
negative lens made of the plastic material in the front group and
the biconvex lens made of the plastic material in the back group
have aspheric surfaces on both surfaces, and the other lenses
except the lenses made of the plastic material are spherical
lenses, respectively.
[0019] In order to achieve the above object, one embodiment of the
present invention also provides an imaging device comprising two
imaging optical systems including a wide-angle lens having a field
angle wider than 180 degrees and an imaging sensor which images an
image by the wide-angle lens, the two imaging optical systems being
combined such that object side lenses are opposed to each other,
and the images by the respective imaging optical systems being
synthesized to obtain an image in a solid angle of 4.pi. radian,
wherein the wide-angle lens for use in each of the two imaging
optical systems is the wide-angle lens according to any one of
claims 1-4, and a distance between an intersection of the
reflection surface and the optical axis of the front group and a
front principle point in the wide-angle lens of one imaging optical
system d1, a distance between an intersection of the reflection
surface and the optical axis of the front group and the front
principle point in the wide-angle lens of the other imaging optical
system d2 and a focal length of the wide-angle lens of each imaging
optical system f satisfy the following condition (4).
16.ltoreq.(d1+d2)/f<21 (4)
[0020] Preferably, the wide-angle lenses for use in the two imaging
optical systems are d1=d2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The accompanying drawings are included to provide further
understanding of the invention, and are incorporated in and
constitute a part of this specification. The drawings illustrate an
embodiment of the invention and, together with the specification,
serve to explain the principle of the invention.
[0022] FIG. 1 is a view describing an optical arrangement of an
imaging device according to an embodiment.
[0023] FIG. 2 is a view illustrating spherical aberration of
Example.
[0024] FIG. 3 is a view illustrating filed curvature of
Example.
[0025] FIG. 4 is a view illustrating coma aberration of
Example.
[0026] FIG. 5 is a view illustrating an OTF feature of Example.
[0027] FIG. 6 is a view illustrating an OTF feature of Example.
DESCRIPTION OF EMBODIMENT
[0028] Hereinafter, an embodiment will be described.
[0029] FIG. 1 is a view illustrating a main portion of an imaging
device.
[0030] In FIG. 1, reference numbers A, B denote imaging optical
systems, respectively.
[0031] Each of the two imaging optical systems A, B includes a
wide-angle lens having a field angle wider than 180 degrees and an
imaging sensor which images an image by the wide-angle lens.
[0032] The imaging optical system A includes a front group having
lenses LA1-LA3, a right angle prism PA constituting a reflection
surface and a back group having lenses LA4-LA7. An aperture stop SA
is arranged on the object side of the lens LA4.
[0033] The imaging optical system B includes a front group having
lenses LB1-LB3, a right angle prism PB constituting a reflection
surface and a back group having lenses LB4-LB7. An aperture stop SB
is arranged on the object side of the lens LB4.
[0034] The lenses LA1-LA3 constituting the front group of the
imaging optical system A are in order from the object side a
negative meniscus lens (LA1) made of a glass material, a negative
lens (LA2) made of a plastic material and a negative meniscus lens
(LA3) made of a glass material.
[0035] The lenses LA4-LA7 constituting the back group are in order
from the object side a biconvex lens (LA4) made of a glass
material, a cemented lens of a biconvex lens (LA5) and a biconcave
lens (LA 6) made of a glass material, and a biconvex lens (LA7)
made of a plastic material.
[0036] The lenses LB1-LB3 constituting the front group of the
imaging optical system B are in order from the object side a
negative meniscus lens (LB1) made of a glass material, a negative
lens (LB2) made of a plastic material and a negative meniscus lens
(LB3) made of a glass material.
[0037] The lenses LB4-LB7 constituting the back group are in order
from the object side a biconvex lens (LB4) made of a glass
material, a cemented lens of a biconvex lens (LB5) and a biconcave
lens (LB6) made of a glass material and a biconvex lens (LB7) made
of a plastic material.
[0038] In these imaging optical systems A, B, the negative lenses
LA2, LB2 of the front group made of a plastic material and the
biconvex lenses LA7, LB7 of the back group made of a plastic
material have an aspheric surface on their both surfaces. The other
lenses made of a glass material are spherical lenses,
respectively.
[0039] The front principal point in each wide-angle lens is set
between the second lens LA2, LB2 and the third lens LA3, LB3.
[0040] The distance between the intersection of the reflection
surface and the optical axis of the front group and the front
principle point is d1 in FIG. 1 in the wide-angle lens of the
imaging optical system A and the distance between the intersection
of the reflection surface and the optical axis of the front group
and the front principle point is d2 in the wide-angle lens of the
imaging optical system B.
[0041] These distances d1, d2 are a distance d in the wide-angle
lens, and the condition (1) 7.0<d/f<9.0 is satisfied.
[0042] The decrease in the parameter d/f of the condition (1) means
the increase in the focal length of the entire system f or the
decease in the distance between the intersection of the reflection
surface and the optical axis of the front group and the front
principle point d.
[0043] If the focal length f is increased, the entire length of the
wide-angle lens on the optical axis is increased. Therefore, if the
focal length is set to an appropriate value in view of downsizing,
the distance d is reduced in that condition.
[0044] Upon the decrease in d, the interval between the lens LA3
(LB3) and the prism PA (PB) is narrowed, so that the limit relative
to the thickness of the lens LA3 (LB3) for ensuring a required
refractive power becomes strict. If the lower limit value of the
condition (1) is lowered, a desired thickness and shape of the lens
LA3 (LB3) can not be obtained, and it becomes difficult to process
the lens LA3 (LB3).
[0045] It is preferable for the imaging optical systems A, B in
FIG. 1 to be close to each other as much as possible in the right
and left direction in FIG. 1, in order to downsize the imaging
device. Since the reflection surfaces are inclined surfaces of the
right angle prism PA, PB, it is effective for the inclined surfaces
to be close to each other as much as possible for downsizing.
[0046] The increase in the parameter d/f of the condition (1) means
the increase in the distance d between the intersection of the
reflection surface and the optical axis of the front group and the
front principle point. This means the increase in the size of the
front group.
[0047] Such an increase in the size of the front group makes it
difficult to downsize the imaging device. In this case, as a method
of preventing the increase in the size of the imaging device due to
the increase in the front group, the imaging optical systems A, B
are arranged to be displaced in the up and down direction in FIG. 1
in a state in which the inclined surfaces of the prisms PA, PB are
arranged to be close to each other.
[0048] However, with this constitution, the optical axes of the
front groups of the wide-angle lenses of the imaging optical
systems are misaligned in the up and down direction in FIG. 1, so
that the effect of the above disparity is increased if such
misalignment goes beyond a certain level.
[0049] If the parameter d/f is smaller than the upper limit of the
condition (1), the increase in the size of the front group can be
maintained within an allowable range while effectively controlling
the effect of the disparity.
[0050] The condition (4) 16.ltoreq.(d1+d2)/f<21 is to control
the condition relative to d/f of the ratio of the distance d and
the focal length f regarding the imaging device. If the parameter
exceeds the lower limit of the condition (4) while controlling the
effect of the disparity, the reflection surfaces of the prisms PA,
PB interfere to each other. If the parameter exceeds the upper
limit, the effect of the disparity can not be ignored.
[0051] The condition (3) nd.gtoreq.1.8 defines that a material in
which the refractive index relative to d-line nd is larger than 1.8
is used as a material of the prisms PA, PB.
[0052] The prisms PA, PB internal-reflect the light from the front
group toward the back group, so that the light path of the imaging
light beam passes in the prism. If the material of the prism has a
high refractive index which satisfies the condition (3), the light
path length in the prism becomes longer than an actual light path
length, and the distance which curves the light beam can be
increased.
[0053] The light path length between the front group and back group
in the front group, prism and back group can be increased longer
than a mechanical light path length; thus, the wide-angle lens can
be downsized.
[0054] By arranging the prisms PA, PB near the aperture stops SA,
SB, a small prism can be used, and the distance between the
wide-angle lenses can be reduced.
[0055] The prisms PA, PB are arranged between the front group and
the back group. The front group of the wide-angle lens has a role
which obtains a light beam having a field angle wider than 180
degrees, and the back group effectively operates for the imaging of
the aberration correction.
[0056] The effects of the manufacturing tolerance and the
arrangement error of the prism can be avoided with the arrangement
of the prisms as described above.
[0057] As described above, the wide-field angle includes the
reflection surface between the front group and the back group, and
the front group and the back group are configured to from a right
angle. With this constitution, the entire length required for
maintaining a high performance wide-angle lens is ensured.
[0058] When using the two wide-angle lenses for an imaging device,
the reflection surface portions are arranged to be close to each
other in the direction orthogonal to the optical axis of the front
group, and the effect of disparity can be effectively reduced.
[0059] Moreover, since the front principle point is set between the
second lens and the third lens in the front group, the size of the
reflection surface can be effectively reduced.
[0060] Accordingly, by using the two wide-angle lenses for an
imaging device, a preferable imaging device having an effectively
reduced disparity can be obtained.
Example
[0061] A specific example of the wide-angle lens will be
hereinbelow described.
[0062] This example illustrates a wide-angle lens for use in the
imaging optical systems A, B of the imaging device illustrated in
FIG. 1. Namely, the two wide-angle lenses for use in the imaging
optical systems A, B are the same, d1=d2.
[0063] In the following example, f denotes a focal length of an
entire system, No denotes an F-number and .omega. denotes a
half-field angle.
[0064] The surface numbers are 1-23 in order from the object side.
These numbers denote a lens surface, incident and emission surfaces
and a reflection surface of a prism, an aperture stop surface, a
filter surface and a light-receiving surface of an imaging
sensor.
[0065] R denotes a curvature radius of each surface, and denotes a
paraxial curvature radius in an aspheric surface.
[0066] D denotes a surface interval, nd denotes a refractive index
of d-line, and .upsilon.d denotes an Abbe's number. An object
distance is infinity. A unit of length is mm.
Example
TABLE-US-00001 [0067] f = 0.75, No = 2.14, .omega. = 190 DEGREES
SURFACE NUMBER R D Nd .nu.d 1 17.1 1.2 1.834807 42.725324 2 7.4
2.27 3 -1809 0.8 1.531131 55.753858 4* 4.58 2 5* 17.1 0.7 1.639999
60.078127 6 2.5 1.6 7 .infin. 0.3 8 .infin. 5 1.834000 37.160487 9
.infin. 1.92 10 .infin.(APERTURE STOP) 0.15 11 93.2 1.06 1.922860
18.896912 12 -6.56 1.0 13 3.37 1.86 1.754998 52.321434 14 -3 0.7
1.922860 18.896912 15 3 0.3 16* 2.7 1.97 1.531131 55.753858 17*
-2.19 0.8 18 .infin. 0.4 1.516330 64.142022 19 .infin. 0 20 .infin.
0.3 1.516330 64.142022 21 .infin. 0.3 22 IMAGING SURFACE
[0068] Surfaces having * (both surfaces of second lens in front
group and both surfaces of final lens in back group) in the above
data are aspheric surfaces.
[0069] An aspheric surface shape is defined by the following known
equation by using an inverse of a paraxial curvature radius
(paraxial curvature) C, a height from an optical axis H, a conical
constant K, and an aspheric coefficient of each order with X as the
aspheric surface amount in the optical axis direction.
X=CH.sup.2/[1+ {square root over (
)}{(1-K)C.sup.2H.sup.2}]+A4H.sup.4+A6H.sup.6+A8H.sup.8+A10H.sup.10+A12H.s-
up.12+A14H.sup.14
[0070] The shape is specified by applying a paraxial curvature
radius, a conical constant and aspheric surface coefficient.
[0071] Aspheric surface data of the above example is as
follows.
Third Surface
[0072] 4th: 0.001612
[0073] 6th: -5.66534e-6
[0074] 8th: -1.99066e-7
[0075] 10th: 3.69959e-10
[0076] 12th: 6.47915e-12
Fourth Surface
[0077] 4th: -0.00211
[0078] 6th: 1.66793e-4
[0079] 8th: 9.34249e-6
[0080] 10th: -4.44101e-7
[0081] 12th: -2.96463e-10
Sixteenth Surface
[0082] 4th: -0.006934
[0083] 6th: -1.10559e-3
[0084] 8th: 5.33603e-4
[0085] 10th: -1.09372e-4
[0086] 12th: 1.80753-5
[0087] 14th: -1.52252e-7
Seventeenth Surface
[0088] 4th: 0.041954
[0089] 6th: -2.99841e-3
[0090] 8th: -4.27219e-4
[0091] 10th: 3.426519e-4
[0092] 12th: -7.19338e-6
[0093] 14th: -1.69417e-7
[0094] In the above aspheric surfaces, for example, -1.69417e-7
means -1.69417.times.10.sup.-7. In addition, 4.sup.th-14.sup.th are
A4-A14, respectively,
[0095] The parameter values of respective conditions are as
follows.
[0096] The parameter value of the condition (1)
[0097] d=d1=d2=6
[0098] f=0.75
[0099] d/f=8
[0100] The parameter value of condition (2)
[0101] DA=8.87
[0102] DB=14.76
[0103] The parameter value of condition (3)
[0104] nd=1.834000
[0105] The parameter value of condition (4)
[0106] (d1+d2)/f=16
[0107] Accordingly, the wide-angle lens and the imaging device of
this example satisfy the conditions (1)-(4).
[0108] The interval between optical axes (interval between front
principle points in up and down direction in FIG. 1) can be reduced
14 mm compared to a wide-angle lens using parallel light paths
without being curved.
[0109] As described above, the light path lengths of the light
beams passing through the periphery and the center of the
wide-angle lens having a filed angle of 180 degrees or more are
changed according to the thickness difference of the lens, causing
deterioration in a performance. Among the three lenses of the front
group in the wide-angle lens in this example, the second lens often
has a thickness difference between the portion near the optical
axis and the periphery of the lens. For this reason, aspheric
surfaces are applied to both surfaces of the second lens as a
plastic lens so as to correct the second lens.
[0110] Moreover, by using the aspheric surfaces on both surfaces of
the final lens of the back group as a plastic lens, the aberrations
generated on the object side of this lens are preferably
corrected.
[0111] Among the four lenses of the back group, the chromatic
aberration is preferably corrected by cementing the second biconvex
lens and the third biconcave lens.
[0112] FIG. 2 illustrates the spherical aberration of the
wide-angle lens of this example. FIG. 3 illustrates the field
curvature of the wide-angle lens of this example.
[0113] FIG. 4 illustrates the coma aberration of the wide-angle
lens of this example.
[0114] FIGS. 5, 6, are views each illustrating an OTF feature. The
horizontal axis illustrates a spatial frequency in FIG. 5 and a
half field angle with degree in FIG. 6.
[0115] As is apparent from these figures, the performance of the
wide-angle lens of this example is extremely high.
[0116] An imaging device to which the imaging device according the
present invention is applied includes an imaging device which
photographs a panoramic image. In particular, this panoramic image
is suitable for an image of horizontal 360-degree, an image of
360-degree attached to a sphere body, namely, an omnidirectionaly
photographed image, or the like.
[0117] Such an imaging device which photographs a panoramic image
may be referred to as an omnidirectional imaging device or an
entire celestial sphere type imaging device. An image photographed
by the imaging device according the present invention can be a
still image or a moving image.
[0118] Although the embodiment including the example of the present
invention has been described above, the present invention is not
limited thereto. It should be appreciated that variations may be
made in the embodiment described by persons skilled in the art
without departing from the scope of the present invention.
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